The chemical explosion was the result of mechanical failure. It wasn't "spontaneous". It was initiated by a spark. And spark induced explosions of fuel vapor have happened countless times.
Name one time when a commerical jet aircraft exploded spontaneously because of a spark incuded explosion of fuel vapor. I have not heard of any other explosions other than the mythical explosion of TWA 800 in the "official" cover-up report. I'll see if I can find you a link to the articles on the military labs testing of jet fuel. Their conclusion was that they couldn't get the fuel to explode at any actual operating temperature in a 747.
How did air get into the fuel tank ?